This invention relates to a mechanical and thermal processing method for production of seamless steel pipes having homogeneous martensitic structure with a combination of high strength and toughness and with minimized distortion, and more particularly to a process for producing such steel pipes at a high thermal efficiency.
In producing seamless steel pipes of high quality with respect to strength and toughness, it has been the prior art practice to carry out either or both of the adjustment of the alloying elements of the steel itself and the heat treatment of the steel pipe of final gage in a manner to control within predetermined limits, the final properties of the steel pipe. Where the heat treatment is employed to control the final properties, the resultant conventional process for producing steel pipes is characterized by the separate and independent application of the forming and heat treating steps. In other words, the pipe forming operation is not correlated to the heat-treating operation involving the quenching and tempering. This permits the use of a heat-treating apparatus as arranged independently of the pipe producing apparatus so that the steel pipe in the as-formed condition is cooled down to room temperature before the application of the heat treatment thereto.
Such an independently operating mechanical and thermal processing method for improving quality characteristics of steel pipes has various disadvantages. One of these is that the heat energy retained in the steel pipe at the forming step is lost with no effect on the heat treating step as the steel pipe is cooled during the time period intervening the forming and heat treating steps. Another disadvantage is biased on the remarkable reduction of the productivity of steel pipes due to the interruption of a production run thereof at a point between the forming and heat treating steps. Still another disadvantage is that the heat treatment requires an additional amount of heat energy as the steel pipe is re-heated from room temperature to and maintained at a temperature at which the heat treatment is performed. This in turn calls for a further increase in the amount of scale produced on the steel pipe surfaces during an elongated cooling time after the pipe-forming operation.
Such scale adhered to the pipe surfaces leads to the reduction of the cooling rate in the quenching step with the resulting slack quenching, which is the main factor in giving rise to increasing the degree of distortion of te quenched pipe.